add CombinationSum and test

src/main/java/com/thealgorithms/backtracking/CombinationSum.java

@@ -0,0 +1,48 @@
+package com.thealgorithms.backtracking;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/** Backtracking: pick/not-pick with reuse of candidates. */
+public final class CombinationSum {
+    private CombinationSum() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    public static List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> results = new ArrayList<>();
+        if (candidates == null || candidates.length == 0) {
+            return results;
+        }
+
+        // Sort to help with pruning duplicates and early termination
+        Arrays.sort(candidates);
+        backtrack(candidates, target, 0, new ArrayList<>(), results);
+        return results;
+    }
+
+    private static void backtrack(int[] candidates, int remaining, int start, List<Integer> combination, List<List<Integer>> results) {
+        if (remaining == 0) {
+            // Found valid combination; add a copy
+            results.add(new ArrayList<>(combination));
+            return;
+        }
+
+        for (int i = start; i < candidates.length; i++) {
+            int candidate = candidates[i];
+
+            // If candidate is greater than remaining target, further candidates (sorted) will also be too big
+            if (candidate > remaining) {
+                break;
+            }
+
+            // include candidate
+            combination.add(candidate);
+            // Because we can reuse the same element, we pass i (not i + 1)
+            backtrack(candidates, remaining - candidate, i, combination, results);
+            // backtrack: remove last
+            combination.remove(combination.size() - 1);
+        }
+    }
+}

src/test/java/com/thealgorithms/backtracking/CombinationSumTest.java

@@ -0,0 +1,29 @@
+package com.thealgorithms.backtracking;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class CombinationSumTest {
+    private static List<List<Integer>> norm(Iterable<List<Integer>> x) {
+        List<List<Integer>> y = new ArrayList<>();
+        for (var p : x) {
+            var q = new ArrayList<>(p);
+            q.sort(Integer::compare);
+            y.add(q);
+        }
+        y.sort(Comparator.<List<Integer>>comparingInt(List::size).thenComparing(Object::toString));
+        return y;
+    }
+
+    @Test
+    void sample() {
+        int[] candidates = {2, 3, 6, 7};
+        int target = 7;
+        var expected = List.of(List.of(2, 2, 3), List.of(7));
+        assertEquals(norm(expected), norm(CombinationSum.combinationSum(candidates, target)));
+    }
+}